1. Field of Invention
This invention pertains to the use of metal-organic frameworks as adsorbents for the separation of composite gasses, and more particularly to adsorbents with a high concentration of alkylamine functionalized sites in a metal organic framework and methods for the separation of a variety of materials based on selective, reversible electron transfer reactions. For example, methods are provided for the separation of individual gases from as stream of combined gases such as CO2 from N2 gases or CO2 from H2 gases from a stream of combined gases.
2. Background
There is a continuing need for the efficient separation of gas mixtures into their component parts in many different industrial processes including energy production and emission reduction. Many gas separations are presently performed on large scales in numerous industrial processes, often at significant cost.
For example, the production of syngas from the conversion of fossil fuels (natural gas, coal, oil, oil shale, etc) or biomass requires the separation of CO2 from H2 and other useful gasses. In this context, the coal or other material is converted into syngas (CO and H2) which subsequently undergoes the water-gas shift reaction to generate CO2 and H2. The hydrogen is used to generate electricity after it is separated from CO2, which can then be prevented from release into the atmosphere. This strategy, called pre-combustion CO2 capture, is advantageous in comparison to other CO2 capture technologies that require separation of CO2 from N2, O2, or CH4 because the differences in size and polarizability between CO2 and H2 can be exploited.
Separation of CO2 from CH4 is also relevant to the purification of natural gas, which can have impurity levels of up to 92% CO2 at its source. Carbon dioxide removal is required for approximately 25% of the natural gas reserves in the United States. Removal of CO2, which is most commonly accomplished using amines to reduce CO2 levels to the required 2% maximum, is conducted at pressures between 20 bar and 70 bar. Carbon dioxide removal is required for approximately 25% of the natural gas reserves in the United States.
Gas separations are also important in post-combustion of fossil fuels for energy production. The combustion of fossil fuels is largely responsible for the increase in the global concentration of CO2 in the Earth's atmosphere, yet fossil fuels will continue to be heavily utilized for energy production during the 21st century.
The development of more efficient processes for capturing CO2 from power plant flue streams is critical for the reduction of greenhouse gas emissions implicated in global warming. Currently, there is significant interest in the development and implementation of technologies that slow CO2 emissions and thus forestall the most severe consequences of global warming. For limiting future CO2 emissions from large, stationary sources like coal-fired power plants, carbon capture and sequestration (CCS) has been proposed. The CCS process involves the selective removal of CO2 from gas mixtures, the compression of pure CO2 to a supercritical fluid, transportation to an injection site, and finally permanent subterranean or submarine storage. For the retrofit of existing power plants, post-combustion CO2 capture is a likely configuration. In this design, fuel is burned in air and CO2 is removed from the effluent. For coal-fired power plants, the largest flue gas components by volume are N2 (70-75%), CO2 (15-16%), H2O (5-7%) and O2 (3-4%), with total pressures near 1 bar and temperatures between 40° C. and 60° C. For post-combustion CO2 capture, maximizing adsorption capacity for CO2 at low pressures is highly desirable. Because the partial pressure of CO2 in flue gas emitted from coal fired power stations is typically between 0.10 and 0.15 bar, the simplest approximation for the capacity of materials being considered is the quantity of gas adsorbed at these lower pressures, not the capacity at 1 bar.
Aqueous amine solutions are currently the most viable adsorbents for carbon capture and are presently used for the removal of CO2 from industrial commodities like natural gas. While a variety of advanced amines are available, 30% monoethanolamine (MEA) in water is the benchmark solvent against which competing technologies are generally compared. The low solvent cost and proven effectiveness make MEA an attractive adsorbent for many applications.
Conventional CO2 capture processes involving the chemisorption of CO2 by alkylamine-containing liquids present several disadvantages, including the considerable heat required to regenerate the liquid, solution boil-off and the necessary use of inhibitors for corrosion control. Therefore, if MEA were to be utilized for carbon capture and sequestration, electricity prices are projected to increase by 86%.
The formation of ammonium carbamate from two MEA molecules and one CO2 molecule endows the scrubber with extremely high selectivity for CO2, but significant energy is required to regenerate the solution. This high regeneration energy cost has two primary components: first, the strong, chemisorptive bond between the carbon dioxide and the amine must be broken; second, a large amount of spectator water solvent must be heated and cooled along with the active amine adsorbent, giving rise to an inefficient system. Because amines are corrosive to plant infrastructure, solutions are typically limited to no more than 30% (w/w) of the amine, and a significant increase in this concentration is not deemed feasible. In addition, solvent boil-off occurs during repeated regeneration cycles consuming the scrubber and increasing costs. The diversion of steam from the electricity generation cycle to the solvent regeneration cycle sharply reduces the net electricity output of the plant, drastically increasing electricity costs. It has been demonstrated that plant efficiency is highly dependent on the solvent regeneration energy that is needed. These limitations represent the most significant obstacles to wider implementation of amine scrubbing technologies for post-combustion carbon capture.
Attempts to address these limitations have focused on the adsorption of CO2 in porous solids such as zeolites and amine-modified silicas via the formation of carbamate or bicarbonate species. The viability of the materials under realistic flue stream conditions requires air and water stability, corrosion resistance, high thermal stability, and high selectivity for CO2 over other components in flue gas. Currently, aqueous amines are used industrially to separate CO2 from gas mixtures with high CO2 partial pressures like natural gas, while some solid adsorbents are used to remove CO2 from mixtures with low CO2 partial pressures. In addition to the separation of combustion gases, there are a number of current industrial processes that utilize liquid or solid adsorbents to remove CO2 from gas mixtures.
Accordingly, there is a need for an efficient methods and materials for selectively separating constituent gases from a stream of gases that can be performed at lower temperatures and pressures than existing techniques. There is also a need for materials and methods that provide selective, reversible electron transfer reactions and associated functions such as catalysis, including oxidation as well as gas storage. The present invention satisfies these needs as well as others and is generally an improvement over the art.